1. Field of the Invention
The present invention relates to a method for use in controlling the emission power of a transceiver which is in communication with another transceiver, for use for example in a wireless communication system.
2. Description of the Background Art
Reference will now be made to FIG. 1 of the accompanying drawings which shows a transceiver 10 in communication with another transceiver 20 via a wireless communication system. Transceiver 10 is for example located in a base station and transceiver 20 in a mobile station. Data are exchanged between the transceiver 10 and the transceiver 20 via a wireless interface, a so-called radio channel. In FIG. 1, the transceiver 20 receives from the transceiver 10 a radio signal references as RC and sends to the transceiver 10 a radio signal with a power referenced as P. The same is for transceiver 10.
The characteristics of radio channels (for example: phase and amplitude) change continuously, due to variations in the geographical environment between a mobile station and a base station. These variations can be separated into free space propagation losses, slow fading and fast fading losses. Free space propagation losses depend on the path length between the transmitter and the receiver and can be modelled by a d−n law where n is a number between 2 and 4 and d is the path length. Slow fading losses are due to shadowing occurring when obstacles, such as buildings, trees, etc., are interposed between the transmitter and the receiver. Slow fading losses are known to generate variations in channel power for movements that are in the order of 10 times the wavelength of the radio signal. They can be modelled by a log normal law the standard deviation σ of which ranges between 4 and 12 dB depending on the kind of environment. Finally, fast fading losses are due to multipath effect in which a signal follows different paths and the resulting received signals recombine at the receiver entrance with different delays, amplitudes and phases. They can be modelled by a Rayleigh distribution. Movements that are in the order of {fraction (1/100)} of the wavelength of the radio signal are sufficient to generate fast fading.
Most telecommunication systems use power control methods to limit interference and power consumption. Power control methods aim to command an emission power of both transceivers as close as possible to the minimum needed for a defined quality of transmission.
Such a method includes, carrying out in an evaluating unit 200 of a transceiver (here the transceiver 20), the steps of measuring the received power of the radio signal RC (or its amplitude) and, on basis of the result of this measurement, of evaluating a power control command PC. The power control command PC is used to command a transmission unit 210 so that it transmits signals with a power P corresponding to the command PC.
Note that the transceiver 10 comprises also such an evaluating unit and a transmission unit.
Due to the time duration (referenced as td in the following description) between the moment of the input amplitude measurement (made in the evaluating unit 200) and the moment of the use of the control command signal PC to command the emission power P (made in the transmission unit 210), the power control methods are based on a measurement and an evaluation which are made during reception and which are used to determine the power to be transmitted during the next emission.
The value of the time duration td is imposed by the system for given periods and thus is known by the considered transceiver.
The emission power P is applied with a certain delay td after the measurement has been made by the evaluating unit 200; hence the channel features can have significantly changed between the evaluation and the application of the PC command. The power control command PC then wrongly compensates the channel variations, particularly in the case of fast fading losses, where the stronger the fading is, the quicker it disappears.